1. Field
This patent specification relates to improved scintillator based radiation detection. More particularly, this patent specification relates to methods and systems for using improved energy calibration and resolution monitoring using intrinsic radiation sources.
2. Background
Scintillation detectors featuring a scintillator crystal and a photodetector (for example a PMT tube) are widely used different industries, and in particular in the field of oilfield services. A common problem in the use of scintillation detectors for nuclear spectroscopy or similar energy sensitive measurements is that the detector response function changes for example with changing environmental conditions. Typically, the sensitivity of the photodetector element will vary with time (drift) and with changing environmental conditions such as temperature and magnetic fields.
Conventionally, the gain of the detectors can be stabilized by using a control circuit that keeps the peak of an external stabilization source signal in the same channel of the multi-channel analyzer. A disadvantage of this method is the need to supply an external stabilization source. Additionally, the external source is often only irradiating part of the crystal which may not give average results. Other techniques are used were the stabilization is based on a measured signal of an external radiation source. Such an external source may not primarily be used for stabilization. Such techniques may use thresholds, windows ratios, or more complex algorithms. The disadvantage is that the radiation may be weak or absent at least part of the time which can result in stabilization loss in particular with changing source strength.
Scintillator materials are widely used to build detectors for measuring X-ray and γ-radiation. Dense materials with high atomic numbers are preferred to measure γ-rays, since the stopping power of the materials increases with these parameters and thus the size of the detector can be reduced without loss of sensitivity. However, many of the heavier scintillator materials have an intrinsic background radioactivity due to the presence of radioactive isotopes in the heavier elements of the crystal matrix. In particular Lutetium has been found to be a valuable constituent in scintillator materials, but suffers from the presence of a radioactive isotope. In large detectors this background count rate might contribute significantly to the maximal achievable count rate and thus negatively affect the precision and accuracy of the measurement. For example lutetium oxyorthosilicate (LSO) has been established as a useful scintillator for medical imaging, but its intrinsic radioactivity affects the count rate in large scintillator crystals. More recently LuAP:Ce and LuAG:Pr have been used as matrix materials for scintillators. Typical intrinsic count rates for material containing a large fraction of Lu are around a few hundred counts per second per cubic centimeter (cm-3s-1). For example a 2″×4″ crystal contains about 200 cm3 of material, so that the count rate reaches around 50,000 s−1. This is about 5-10% of the count rate capability of a fast conventional detector and thus creates a loss in statistical precision of several %. Intrinsic radioactivity is therefore conventionally regarded as a disturbance.